Problem: Ashley is 2 times as old as Christopher. 35 years ago, Ashley was 9 times as old as Christopher. How old is Christopher now?
Solution: We can use the given information to write down two equations that describe the ages of Ashley and Christopher. Let Ashley's current age be $a$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $a = 2c$ 35 years ago, Ashley was $a - 35$ years old, and Christopher was $c - 35$ years old. The information in the second sentence can be expressed in the following equation: $a - 35 = 9(c - 35)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to use our first equation for $a$ and substitute it into our second equation. Our first equation is: $a = 2c$ . Substituting this into our second equation, we get: $2c$ $-$ $35 = 9(c - 35)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $2 c - 35 = 9 c - 315$ Solving for $c$ , we get: $7 c = 280.$ $c = 40$.